I. Field of the Invention
The present invention relates generally to the measurement of dissolved gases. More particularly, the present invention relates to a method and system for quantitative measurement of dissolved gases, particularly dissolved gases present in groundwater.
II. Background of the Invention
Dissolved gases in groundwater can originate from equilibration with the atmosphere, incorporation of excess air during unsaturated zone migration, and production of radiogenic, chemical, or biological processes. Typical groundwater gases include N2, N2O, NO, O2, CO2, CH4, and H2S; and smaller concentrations of noble gases such as Ar, He, Kr, Rn, Ne, and Xe. While some are relatively inert in aquatic systems and can serve as hydrogeological tracers, others are actively involved in significant biogeochemical processes, playing a major role in the chemical evolution of groundwater and global geochemical cycles. Thus, the quantitative measurement of dissolved gases in groundwater can provide insight into transport and biogeochemical processes in aquifers.
The routine use of dissolved gas concentrations is becoming increasingly common in a number of fields such as geochemical exploration, seismology, paleoclimatology, age-dating of young groundwater, groundwater tracers, environmental assessment of oil and gas production, and measurement of volatile organic compound contaminants (e.g., gasoline constituents and chlorinated solvents). Monitored natural attenuation is becoming an increasingly popular remediation strategy at industrial sites with soil and/or groundwater contamination by organic contaminants (e.g. oil and gas, chlorinated solvents, etc.). Accurate and reliable data are required to demonstrate to regulators that sufficient biodegradation occurs. Regulators look for both a decrease in contaminant concentrations and evidence that these contaminants are degrading, ideally to CO2 and CH4 degradation end-products. Measurement of these gases is also valuable for interpretation of sub-surface chemical and biogeochemical processes and robust mass balance calculations.
Despite their value, dissolved gas analyses are often under-utilized in geochemical investigations, largely because routine sampling and analytical procedures are not available. Sampling is often onerous and sample integrity compromised by degassing during sampling and manipulation for analysis.
The most commonly used sampling protocol involves pumping groundwater into a vial (with minimal atmospheric contact) and transporting the water sample to the laboratory where headspace partitioning and gas chromatographic analysis of the headspace are conducted. Degassing is often caused by decreased hydrostatic pressures during pumping, with bubbles often visible in clear pump tubing. This causes gas sample loss and a negative sample bias. Selective partitioning of more volatile gases can also result in variable sample bias. The use of peristaltic pumps in dissolved gas sampling can result in a loss of up to 10-20% of the sample due to degassing. Also, pumping often alters the natural chemical gradients and produces vertically mixed water samples from different layers of the aquifer. In addition to gas exsolution during sampling, traditional gas-sampling methods require the extraction of gases by headspace partitioning prior to gas chromatographic analysis. This procedure is time-intensive and has the potential for loss of sample due to manipulation. Ideally, groundwater gas concentrations should be sampled under insitu hydrostatic and dissolved gas pressures to ensure no degassing occurs.
Wilson (Wilson et al., Journal of Hydrology 1990, 113, 51-60) and Castro (Castro et al., Water Resources Research 1998, 34, 2467-2483) describe attempts to eliminate atmospheric degassing by isolating water samples in crimped lengths of copper tubing. Others have described a sophisticated apparatus with an evacuated vacuum flask attached to an evacuated side arm (Pearson et al., Geochimica Cosmochimica Acta 1978, 42, 1799-1807; and Dunkle Shapiro et al., Water Resources Research 1993, 29, 3837-3860). The use of gas tight syringes to sample water directly from pump tubing at the surface to eliminate atmospheric contamination has also been described (McCarthy et al., Water Resources Research 1993, 29, 1675-1683; and Theirrin et al, Ground Water 1995, 33, 469-475), however, samples still experience degassing while they are pumped from the groundwater zone to ground surface. Still further, an ampoule fusing process has been developed by United States Geological Survey researchers for collection of gas samples. Relatively sophisticated and specialized equipment is required to extract the sample from tubing or ampoules prior to analysis.
More recent methodologies eliminate atmospheric contact and attempt to eliminate degassing of the sample due to depressurization caused by pumping to ground surface. A down-hole variation on the copper tube sampling method has been used, and water samples for CH4 analysis have also been collected using down-hole syringes.
Still more recently, a number of in situ sampling prototypes have been developed, including samplers incorporating water filled diffusion cells, however these methods do not eliminate the need for head-space partitioning in the laboratory with associated expense and potential for lack of accuracy.
Sorbent samplers, containing an absorbent material in a gas-filled chamber surrounded by a water-impermeable/vapour permeable membrane, have also been developed for monitoring volatile and semi-volatile compounds in water. Sorbent samplers are limited to volatile organic compound analysis and often require a solvent extraction and calibration step in the lab prior to analysis. For example, U.S. Pat. No. 5,922,974 describes an apparatus for extracting soil gases from the earth and concentrating the gases within a resin or molecular sieve. The resin must be transported to a laboratory and heated to release and analyze the collected gas.
Simple gas-filled diffusion cells have been used in measuring dissolved gas concentrations. For example, ping-pong balls covered with latex tubing have been utilized in analysis of helium in lake sediments (Dyck and Silva, Journal of Geochemical Exploration 1981, 14, 41-48; and Stephenson et al., Journal of Hydrology 1994, 154, 63-84). Although this approach provides in-situ gas sampling, the gas sample must later be transferred into a gas-tight syringe for transport to the analytical laboratory.
In summary, a variety of in situ passive-diffusion gas sampling methods have been used historically in surface water bodies and more recently in surface water-sediment interfaces. In addition to being more efficient in the field, these methods eliminate the need for a headspace partitioning step in the lab (and the associated lack of accuracy). However, passive diffusion sampler use in ground water has been limited as such samplers require either extensive machining, sample manipulation after being brought into the laboratory, or have limited depths at which they can be employed.
Samplers have been developed which include on-site analysis systems. For example, U.S. Pat. No. 6,272,938 provides a device for monitoring of Volatile Organic Compounds in groundwater, which has a gas sensor within the device, or which is connected to a gas chromatograph to provide immediate analysis of groundwater contaminants. U.S. Patent Publication 2004/0129058 provides a vapour trap for installation within the floor of a facility to monitor the accumulation of VOC's. The vapour trap is continuous with an organic vapour analyzer, and provides a sampling pump for drawing gas samples from the vapour trap. These devices are not intended to measure dissolved gas concentrations.
It is, therefore, desirable to provide an in situ dissolved gas sampler that simplifies the steps and/or improves efficiency of sample collection, sample storage, and analysis. More specifically, it is desirable to provide an efficient, cost-effective gas sampler that does not compromise sample integrity, require on-site analytical equipment, pumps, or the installation of semi-permanent or permanent equipment at the test site.